Study of the very high energy gamma-ray emission from supernova remnants with VERITAS: two test cases: Cassiopeia A and IC 443

Abstract

The research in this thesis is concerned with the study of gamma-ray emission from supernova remnants (SNRs). In particular, I have performed an analysis of the data accumulated from VERITAS observations of two SNRs: Cassiopeia A and IC 443. VERITAS is an array of four 12 m telescopes located in Arizona at an altitude of 1268 m. The current configuration of the VERITAS observatory is sensitive to gamma-ray photons with energies from 85 GeV to 30 TeV. I have also performed data analysis in the high-energy (HE) gamma-ray domain using publicly available Fermi-LAT data on Cassiopeia A. The HE/VHE emission from supernova remnants arises from the interaction between high energy cosmic ray particles accelerated at the SNR shocks and the surrounding medium. Therefore, gamma-ray emission can be used as a powerful probe for testing SNRs as a potential source of Galactic cosmic rays. ☐ Chapter 1 of this thesis gives an introduction to gamma-ray astronomy and the instruments used for detecting HE/VHE gamma rays. This chapter also summarizes the population of very high-energy (VHE) gamma-ray sources which have been detected so far. Chapter 2 discusses the general imaging atmospheric Cherenkov technique for detecting VHE gamma-ray showers from the ground. This is followed by Chapter 3, in which a comprehensive description of VERITAS observatory is presented. Chapter 4 describes the analysis techniques used for analyzing the data from the VERITAS array. Chapter 5 includes a broad description of SNRs in the HE/VHE gamma-ray domain. In this chapter, a general classification of SN explosions and their dynamical evolution is discussed. A simplified mathematical treatment of diffusive shock acceleration theory is presented, followed by a description of gamma-ray production mechanisms at HE/VHE energies. The final part of Chapter 5 discusses the gamma-ray emission from young and middle-aged SNRs. ☐ Chapter 6 provides a detailed description of the results from Cassiopeia A obtained by analyzing data from Fermi-LAT and VERITAS. The second part of Chapter 6 discusses the interpretation of the HE/VHE gamma-ray emission result in the context of multi-wavelength data. The final Chapter of the thesis presents the detection of extended gamma-ray emission from a middle-aged SNR, IC 443. This SNR is interacting with molecular clouds and evolving in a very heterogeneous environment. To understand the possible origin of gamma-ray emission from IC 443, I present the results of some modelling of the system.